This invention relates to electronic circuits such as variable loopwidth filters wherein different capacitors are switched into and out of operation and, more particularly, to an apparatus and method for reducing transients that result from this switching of capacitors.
The present invention finds particular application in logging-while-drilling of earth boreholes. Logging-while-drilling involves the transmission to the earth's surface of downhole measurements taken during drilling, the measurements generally being taken by instruments mounted just behind the drill bit. One scheme for transmitting the measurements to the earth's surface utilizes drilling fluid within the borehole as a transmission medium for acoustic waves modulated with the measurement information. Typically, drilling fluid or "mud" is circulated downward through the drill string and drill bit and upward through the annulus defined by the portion of the borehole surrounding the drill string. This is conventionally done to remove drill cuttings and maintain a desired hydrostatic pressure in the borehole. In the technique referred to, a downhole acoustic transmitter, known as a rotary valve or "mud siren", repeatedly interrupts the flow of the drilling fluid, and this causes an acoustic carrier signal to be generated in the drilling fluid at a frequency which depends upon the rate of interruption. The acoustic carrier is modulated as a function of downhole digital logging data. In a phase shift keying ("PSK") modulating technique, the acoustic carrier is modulated between two (or more) phase states. Various coding schemes are possible using PSK modulation. In a "non-return to zero" coding scheme, a change in phase represents a particular binary state (for example, a logical "1"), whereas the absence of a change of phase represents the other binary state (for example, a logical "0"). The phase changes are achieved mechanically by temporarily modifying the interruption frequency of the mud siren to a higher or lower frequency until a desired phase lag (or lead) is achieved, and then returning the mud siren to its nominal frequency. For example, if the nominal frequency of the mud siren is 12 Hz., a phase change of 180.degree. can be obtained by temporarily lowering the frequency of the mud siren to 8 Hz. for 125 milliseconds (which is one period at 8 Hz. and one and one-half periods at 12 Hz.) and then restoring the mud siren frequency to 12 Hz. It is readily seen that a 180.degree. phase shift could also be achieved by temporarily increasing the mud siren frequency for an appropriate period of time (i.e., to obtain a desired phase lead), and then returning to the nominal frequency.
The modulated acoustic signal is received uphole by one or more transducers which convert the acoustic signal to an electrical signal. It is then necessary to recover the digital information which is contained in the modulation of the received signal. Briefly, this is achieved by first processing the received signals to extract the carrier signal. The reconstructed carrier is then used to synchronously demodulate the modulated electrical signal.
In the type of system described, a carrier tracking loop is typically employed at the receiver, the purpose of the tracking loop being to lock onto the carrier of the received signals and to produce timing signals that can be used in the demodulation process. It is desirable to acquire a locking onto the carrier as quickly as possible so as to avoid possible loss of information. It is also desirable, once lock is achieved, to have a tracking loop which will be relatively stable; i.e. not adversely affected by short term error component signals in the loop at various frequencies. These two objectives are somewhat at odds, since relatively fast acquisition of lock requires a relatively wide loopwidth whereas stability of the loop would generally dictate a relatively narrow loopwidth. It is known that loopwidth can be manually varied once lock has been achieved, but this technique is not particularly convenient. Also, in the type of logging-while-drilling appratus described above, where relatively low frequency acoustic signals are employed, practical problems arise when attempting to vary the loopwidth of the carrier tracking loop. In particular, the varying of loopwidth generally involves the switching of different capacitors into the loop filter circuit and, at the same time, modifying the loop gain factor. At the frequencies of interest, the capacitors in the circuit generally have relatively large values and are implemented using electrolytic capacitors which provide relatively large capacitance without the undue size which is typical of non-electrolytic capacitors. When a previously inactive capacitor is switched into the circuit, a problem arises due to introduction of an offset voltage which results from the previous voltage across the new capacitor not corresponding to the voltage applied thereacross once it is switched into the circuit.
It is an object of the present invention to provide an improved variable loopwidth filter and circuit for reducing transients when switching capacitors.